Bubble measuring instrument and method

ABSTRACT

Method and apparatus are provided for a non-invasive bubble measuring instrument operable for detecting, distinguishing, and counting gaseous embolisms such as bubbles over a selectable range of bubble sizes of interest. A selected measurement volume in which bubbles may be detected is insonified by two distinct frequencies from a pump transducer and an image transducer, respectively. The image transducer frequency is much higher than the pump transducer frequency. The relatively low-frequency pump signal is used to excite bubbles to resonate at a frequency related to their diameter. The image transducer is operated in a pulse-echo mode at a controllable repetition rate that transmits bursts of high-frequency ultrasonic signal to the measurement volume in which bubbles may be detected and then receives the echo. From the echo or received signal, a beat signal related to the repetition rate may be extracted and used to indicate the presence or absence of a resonant bubble. In a preferred embodiment, software control maintains the beat signal at a preselected frequency while varying the pump transducer frequency to excite bubbles of different diameters to resonate depending on the range of bubble diameters selected for investigation.

This application is a division of application Ser. No. 09/498,440, filedFeb. 4, 2000, now U.S. Pat. No. 6,408,679.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 U.S.C. 2457).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus and methods for measuringbubbles in fluids and tissues and, more specifically, to apparatus andmethods for detecting, sizing, and counting gaseous emboli in anon-invasive manner.

2. Description of Prior Art

In-vivo measurement of the size and number of bubbles in fluids andtissues may be used to prevent, diagnose, and/or treat many potentiallyserious medical conditions such as, for example only, decompressionsickness or stroke following cardiopulmonary bypass procedures. In-vitroor out of the body measurement of bubbles is also useful in connectionwith medical equipment that involves the flow of fluids into or out ofthe body. Emboli of various types may occur in the body for many medicalreasons. Detecting and/or distinguishing gaseous emboli from other typesof emboli is highly desirable so that appropriate medical managementdecisions can be made. Emboli may consist of formed elements such asblood clots, platelet aggregates, or other particulate matter such aspieces of atherosclerotic plaque or fat. Emboli may also consist of gasbubbles introduced to the blood vessels through injection, surgicaltechniques, cavitation at prosthetic valves, or decompression orcompression to lower or higher atmospheric pressures.

In-vivo measurements of bubbles are especially useful with respect todecompression sickness. Decompression sickness poses a risk of seriousinjury or death to aviators, astronauts, divers, and others who areexposed to varying environmental pressure conditions. NASA, Air Force,Navy, and civilian personnel rely on pressure suits, controlledbreathing mixtures, and operating procedures to maintain “acceptable”environmental conditions to prevent decompression sickness. These“acceptable” conditions are determined empirically, based onexperimental observations of decompression sickness and its precursors.The symptoms of decompression sickness are attributed to the presence ofgas bubbles, comprised mostly of nitrogen, in vascular and extravasculartissue. In vascular tissue, these bubbles can lodge or embolize invessels in the pulmonary or systemic circulation systems, resulting in arange of pathology which is included in decompression sickness. Thesebubbles are formed due to local supersaturation of nitrogen uponreduction of ambient pressure or possibly upon warming from ahypothermic condition. The formation of bubbles and the onset ofdecompression sickness, which do not necessarily coincide, are highlyvariable and depend on a large range of factors including duration andmagnitude of ambient pressure excursions, exercise, hydration, rate ofchange of pressure, hypoxia, temperature, age, infection, fitness,fatigue, previous injury, sex, and body fat.

In addition to decompression sickness, embolic events associated withthe use of cardiopulmonary bypasses have been a serious concern. Thereare an estimated 700,000 cardiopulmonary bypass procedures performedannually in the U.S. In prospective studies of postoperativeneurological dysfunction following cardiopulmonary bypass, the incidencerate is as high as 30% to 60%. The incidence of stroke followingcardiopulmonary bypass is 1% to 5%. It is generally accepted that theseeffects are a consequence of microembolism, and/or compromise ofcerebral blood flow. Emboli associated with cardiopulmonary bypass canbe comprised of biological material, such as oxygen or nitrogen. Thesource of blood cell aggregates and thrombi is typically an activationof the thrombogenic cascade by blood interaction with a foreign surface,or they may be introduced with transfused blood. The sources of gaseousemboli include the blood oxygenation system and cavitation in thepumping systems.

Another major source of gaseous emboli is so-called “surgical air”,which can be introduced during cardiotomy for procedures like valvularand septal repair in the heart. These bubbles are of particular concern,because they contain air (primarily nitrogen) and are much less solublein blood and tissue than oxygen bubbles. “Surgical air” has also beenassociated with neurological dysfunction in major organ transplantsurgeries, such as liver transplants.

Gaseous emboli can also be generated in the body as a result ofcavitation associated with artificial heart valves. These devices alsopotentially create thrombotic emboli, and as a result there is a needfor instrumentation which can distinguish between the two types ofevents to aid device development and to aid diagnosis.

An improved ability to monitor for vascular and extravascular bubbleswould have a significant impact on the ability to prevent and minimizedecompression sickness and embolic pathology. In particular, better dataon the early occurrence of bubbles, their size, and their composition(gaseous/non-gaseous) will permit less restrictive operational anddesign criteria to be developed for the prevention of decompressionsickness in astronauts, aviators, and divers by permitting directobservation of the important variable of bubble size duringdecompression events. Direct monitoring of operational personnel in highrisk decompression sickness circumstances would provide a quantitativeindication to provide much more accuracy as to their proximity to theonset of symptomatic decompression sickness.

Improved monitoring would aid in therapy, recovery, and survival ofpatients being treated for decompression sickness and gaseous embolismby providing the first quantitative information about the size of thebubbles which are giving rise to their pathology. It would be highlydesirable to provide for direct monitoring of the presence and size ofgaseous bubbles such as gaseous emboli during and after surgicalprocedures with high likelihood of emboli introduction, such ascardiopulmonary bypass, with the goal being a subsequent decrease in therate of embolic complications.

More generally, improved monitoring would provide clinicians with earlywarning of the introduction, size, and composition of emboli, allowingfor better informed therapeutic approaches to be used. As well,biomedical researchers would have an improved ability to classify andquantify emboli produced by artificial heart valves and cardiopulmonarybypass machines.

To date, the detection of emboli, both gaseous and non-gaseous, has beenlargely accomplished through the use of Doppler ultrasound. Thistechnique tells the observer whether there are bubbles or emboli presentand provides an indication as to how many are present based on the rateof detection. The Doppler technique is only able to detect emboliflowing with sufficient speed in large vessels, when the direction ofmotion of the flow and the orientation of the acoustic beam are in arestrictive range. Doppler techniques have virtually no ability toquantify the size of the bubbles, observe bubbles in non-vascular tissueor in slow flowing microvessels, and have limited usefulness inclassifying emboli as gaseous or non-gaseous. These are seriouslimitations with regard to detection and classification of: (1)decompression sickness precursor bubbles, (2) emboli during surgicalprocedures, and (3) emboli generated by artificial heart valves.

The following patents disclose attempts to solve the above discusseddifficult problems and related problems over the last two decades.

U.S. Pat. No. 5,441,051, issued Aug. 15, 1995, to Hileman et al.,discloses a method and apparatus for ultrasonically detecting an embolusin blood flow, including an ultrasound transducer for transmittingultrasound pulses into the blood flow being interrogated and receivingreflections from acoustic impedance changes in the body. The reflectedsignals are converted to an electronic signal representation which issubsequently processed to detect and classify emboli in the blood flow.A short duration, broad bandwidth ultrasound signal is used to preservethe polarity of the reflected signal. The polarity is then used toclassify the emboli based on a positive or negative reflectioncoefficient. Emboli having a negative reflection coefficient areclassified as either gaseous or fat particles, and emboli having apositive reflection coefficient are classified as solid particles. Theemboli can be further classified based on the amplitude of the reflectedsignal, or designated features of the time waveform or FFT of thereflected signal.

U.S. Pat. No. 5,348,015, issued Sep. 20, 1994, to Moehring et al.,discloses a noninvasive means for detecting, counting, andcharacterizing emboli moving through the arterial or venous circulation.An ultrasonic transducer is applied to the skin or other tissues of thesubject at sites such as over the temporal bone on either side of thehead of the subject, on the neck, on the chest, the abdomen, arm, leg,within the esophagus, or surgically exposed organs or blood vessels.Using standard ultrasonic Doppler techniques, Doppler-shifted signalsare located which are proportional to the blood flow velocity in theblood vessel(s) of interest. Spectral analysis is performed on thereceived signal using the fast Fourier transform or other appropriatetechnique to determine the frequency components in the Doppler shiftspectrum. Further analysis of the spectra is used to delineate andcharacterize Doppler shift signals due to blood from Doppler shiftsignals due to emboli having a variety of compositions.

U.S. Pat. No. 5,198,776, issued Mar. 30, 1993, to Kenneth L. Carr,discloses an apparatus an method for detecting the presence ofincidental bubbles in liquid flowing in a tube. The system monitors theamplitude of microwave radiation from the liquid and recognizes whenthat amplitude drops in a manner characteristic of the presence of abubble.

U.S. Pat. No. 5,103,827, issued Apr. 14, 1992, to George H. Smith,discloses a method and apparatus for distinguishing ultrasound signalsreturned from bubbles and particles moving in a fluid from signals dueto ultrasound transducer motion that monitors the receiving ultrasoundsignal for signals which are of much larger amplitude than the signalsobserved when no gas bubbles or particles are present. When a largeamplitude event is detected, the maximum amplitude of the forward flowsignal (that is, the positive frequency portion of the power spectrum)is compared to the maximum amplitude of the reverse flow signal(negative frequency portion of the power spectrum). If these maxima aresignificantly different in amplitude, the event is counted as a bubble.If the maximum amplitudes of the forward and reverse flow signals arecomparable, the event is classified as a motion artifact. Displays ofthe spectra are marked whenever an event is counted as an air orparticulate emboli so as to call attention to the event and, optionally,to generate an audible or visual alarm.

U.S. Pat. No. 4,689,986, issued Sep. 1, 1987, to Carson et al.,discloses a system for detecting gas bubbles in a specimen utilizing atransducer which produces pulses, illustratively of ultrasonic acousticenergy, having predetermined frequency characteristics. A first pulsehas an increasing frequency with time, and a second pulse has adecreasing frequency with time. Imaging arrangements, which may beformed of ultrasonic transducers, produce images of the region withinthe specimen after exposure to each such pulse. In one embodiment, agrowth transducer array is utilized for dramatically increasing the sizeof the bubbles, which array is formed of a plurality of transducerswhich are moved with respect to the specimen and which have respectivefrequency characteristics over different frequency ranges. Thus, bubbleradius is successively increased as each bubble is exposed to theacoustic energy from each such transducer within the growth transducerarray. The present invention of Carson et al can be used to reduce thecavitation threshold of bubbles, particularly in the vicinity of tumors,or to increase the temperature in the bubble-containing region.

U.S. Pat. No. 4,657,756, issued Apr. 14, 1987, to Rasor et al.,discloses that microbubbles are formed in a liquid, e.g., blood in orderto alter the transmission characteristics thereof to electromagnetic andsonic waves transmitted therethrough, by dissolving therein a solidparticulate material, preferably as a suspension in a carrier liquid inwhich the particulate material is at least temporarily stable, theparticles of which are substantially free of microbubbles and have aplurality of gas-filled voids in fluid communication with the surface ofthe particles and providing nuclei for microbubble formation and theratio of the mass of the particles to the volume of gas in the voids issufficient to render the liquid in which the particulate material isdissolved supersaturated with respect to the gas in the voids in thearea of the liquid surrounding the microbubbles when they are formed.

U.S. Pat. No. 4,483,345, issued Nov. 20, 1984, to Hirohide Miwa,discloses a system for measuring from the outside of a living body thepressure within the heart of the pressure of any portion which does notallow a measurement by the direct insertion of a pressure measuringsensor. This system provides a method of measuring the pressure of theobject by generating fine bubbles through cavitation, applying alow-frequency ultrasonic wave to the medium, and then detecting thebubbles which are generated with a system for detecting the high orlow-frequency harmonics due to the bubbles or a higher frequencyultrasonic wave applied to the medium.

U.S. Pat. No. 4,459,853, issued Jul. 17, 1984, to Miwa et al., disclosesa probe which comprises a plurality of ultrasonic transducer elements,and is so arranged as to be capable of simultaneously transmitting andreceiving ultrasonic beams of plural frequencies. Means is provided forchanging the shapes of the effective acoustic field of the ultrasonicbeams of each a predetermined number of frequencies by selectivelyoperating the ultrasonic transducer elements or interchangingtransducers. The shapes of the effective acoustic fields of theultrasonic beams of the plural frequencies are made substantiallycoincident in accordance with the range of distance from the probe.Thereby, the measuring of the tissue or the like with coincident shapedbeams of plural frequencies can be realized.

U.S. Pat. No. 4,290,432, issued Sep. 22, 1981, to Stephen Daniels,discloses a decompression bubble detector which comprises a pulsedultra-sound transmitter/receiver which is scanned across a cross-sectionof tissue and the total number of pulse echoes received in a preselectedtime interval is recorded. Changes in the total number of pulse echoesrecorded in successive time intervals are used to monitor thedecompression. A single transducer is scanned by means of a driveneccentric cam and a cam follower. A sin/cos potentiometer generates asignal related to the angular position of the transducer connected to adelay so that pulse counting can be arranged to coincide with thepassage of the transducer across the target.

U.S. Pat. No. 4,152,928, issued May 8, 1979, to Richard A. Roberts,discloses a system utilizing a bank of frequency staggered band-passfilters spanning the range in which fat emboli are known to occur whichis used for the early detection of fat emboli in blood.

U.S. Pat. No. 4,015,464, issued Apr. 5, 1977, to Miller et al.,discloses an apparatus for sensing particles in a fluid medium whichcomprises an ultrasonic resonant cavity for containing a fluid medium. Afirst transducer on one side of the cavity continuously propagatesthereacross ultrasonic compressional waves whose phase and amplitude areperturbed by the presence of particles in the fluid medium. A secondtransducer positioned on the opposite side of the cavity from the firsttransducer substantially parallel to and in registry therewith receivesthe ultrasonic waves and converts them to rf electric waves of the samefrequency, the rf electric waves having their phases and amplitudesmodulated in response to any perturbations in the ultrasonic waves. Therf waves are amplified and fed back to the first transducer thereby toestablish an oscillatory circuit. An attenuator in the oscillatorycircuit causes its operation to be marginally oscillatory whereby smallchanges in the amplitude of the rf waves caused by any perturbations inthe ultrasonic waves produce relatively large changes in the amplitudethereof. A detector responsive to perturbations in the rf wavedemodulates the amplified rf wave to produce signals indicative of thepresence of particles in the fluid medium. Thus, enhanced sensitivity tosmall changes in the ultrasonic properties of the fluid medium caused bythe presence of particles therein is achieved.

U.S. Pat. No. 3,974,683, issued Aug. 17, 1976, to Roger Martin,discloses an apparatus for ultrasonic testing which comprises a pulsedultrasonic transducer, means for detecting echoes from bubbles in aliquid and means for determining the volume of the bubbles.

U.S. Pat. No. 3,974,681, issued Aug. 17, 1976, to Jerry Namery,discloses the mode of operation by ultrasonic through-transmission and adetector preferably employed for detecting air bubbles in intravenousfeeding tubes to prevent air embolism. Transmission of sound from thetransmitter, via the sensor head, to the receiver of the detector isdependent upon the existence of a fluid within the tubing. Acousticlosses, operating frequency, and the distance between transmitter andreceiver are optimized to permit constructive-interference of energytransmitted to and reflected from the receiver, resulting in a partialstanding wave as in a resonant cavity. If an air bubble passes throughthe sensor head, a large acoustic discontinuity occurs, causingultrasound to scatter and reflect from its normal path. These lossesallow little ultrasonic energy to couple to the receiver. The sensorhead includes spaced oppositely disposed cylindrical sound pipe segmentshaving facing tubing accommodating recesses, and respectively connectingto the transmitter and receiver. Sound pipe segments have a markedlyhigher refractive index in comparison with the feeding tubing and itsliquid contents causing ultrasound energy to focus towards the center ofthe feeding tube, thereby yielding greatest sensitivity to transmissionlosses through the fluid within the feeding tube.

The above cited prior art does not provide an in vivo means for sizingand counting the bubbles of any particular specific size or forselectable ranges of bubble sizes as is desirable for many medicalpurposes. Consequently, there is a strong need within the biomedicalresearch community for the noninvasive, bubble sizing instrumentdisclosed herein. Those skilled in the art have long sought and willappreciate the present invention that addresses these and otherproblems.

SUMMARY OF THE INVENTION

A method is provided for monitoring a selected volume for gaseousbubbles comprising steps such as producing a first acoustic signal at afirst frequency and producing a second acoustic signal having a secondfrequency higher than the first frequency. The second acoustic signal isproduced in a pulsed mode such that the pulses are produced at arepetition frequency. The first frequency and the repetition frequencyare selectable to produce a beat signal with a selected frequency. Thebeat signal is monitored at the selected frequency to detect thebubbles.

The first frequency and the repetition frequency are selected such thatthe repetition frequency is not equal to the first frequency divided byan integer. The first frequency may preferably be varied to monitor arange of bubble sizes. As the first frequency is varied, the repetitionfrequency may be continually adjusted to maintain the beat signal at theselected frequency.

The second frequency is preferably kept constant. When using a singletransducer, the transducer may be monitored for the beat signal betweenpulses of the second acoustic signal.

A high pass filter may be used to eliminate signals below the selectedfrequency for detecting the beat signal. As well, a low pass filter maybe used to eliminate signals above the selected frequency for detectingthe beat signal.

The first and second acoustic signals are directed at the selectedvolume in which bubbles are to be measured. A signal response isdetected from the selected volume. The first frequency and therepetition frequency are preferably controlled such that the repetitionfrequency is not equal to the first frequency divided by an integer asthe first frequency varies. A beat signal is detected related to thepresence of a bubble in the selected volume. The repetition frequency ispreferably controlled to maintain the beat signal at a constant beatfrequency as the first frequency varies.

A rate for varying the first frequency may be selected such that therate is related to the time required for monitoring the range of bubblesizes to be detected. Selecting the rate for varying the first frequencyfurther comprises selecting a frequency sweep increment such that thepump frequency is incremented in steps through a range of frequenciesrelated to the range of bubble sizes to be detected. The bubble size maybe determined based on the first frequency.

An apparatus for monitoring bubbles in a selected volume comprises apump transducer and a controller for the pump transducer operable forproducing a signal at a first frequency from the pump transducer. Thefirst frequency may be selectable over a range of frequencies. An imagetransducer is provided and a reference signal generator is used forproducing a reference signal at a second frequency higher than the firstfrequency. A pulser produces a pulsed signal output from the imagetransducer at the second frequency. A receiver may be used for detectinga return signal from the image transducer and a multiplier may be usedfor multiplying the return signal with the reference signal to produce amultiplier output signal.

A first low pass filter is preferably used for filtering the multiplieroutput signal to produce a first filtered signal. A sample and holdcircuit receives the first filtered signal to further detect a beatsignal. A second lowpass filter and a highpass filter may be used forproducing the beat signal. A level detector which may be softwarecontrolled may be used for detecting a bubble from the amplitude of thebeat signal response. A pulser controller may be used for controlling arepetition frequency of the pulsed signal output. The pulser controlleris preferably software controlled for selecting of the repetitionfrequency. The pulser controller may vary the repetition frequency basedon the first frequency. Preferably, the pulser controller is programmedfor varying the repetition frequency to maintain a constant frequency ofa beat signal contained in the return signal.

Preferably a video bubble sizing apparatus is provided for verifyingoperation of the ultrasonic bubble monitor in vitro and comprises avideo camera with software for capturing video images, a sight tubecontaining bubbles, and a strobe for producing separate video framesshowing bubbles. A video monitor may be used for viewing the separatevideo frames with bubbles therein. The software may be operable forproviding statistically independent frames such that the same bubble isnot measured twice in the separate video frames.

A tissue phantom is preferably used for simulating in-vivo bubbles to beobserved ultrasonically in a tube through which bubbles are entrained. Ahousing is provided having a covering of synthetic material to simulateskin. A rod may be disposed within the housing for simulating bone; anda gel may be disposed within the housing for simulating tissue. Thevideo system for viewing bubbles in the tube verifies results of thein-vitro bubbles observed ultrasonically.

One object of the present invention is to provide an improved instrumentand method for non-invasively monitoring bubbles.

Another object of the present invention is to size bubbles over a rangethat includes at least the gaseous emboli in the range of 40 ?m to 400?m although detection of gaseous emboli outside this range may also beuseful and may be accomplished using the present invention.

Yet another objective of the present invention is to verify the bubblemonitoring instrument performance by providing an independent means formeasuring the size of bubble populations.

Yet another objective of the present invention is to provide an in-vitromethod to test operation of the present invention in a manner thatsimulates in-vivo operation and provides for verification of operationand instrument accuracy.

Any listed objects, features, and advantages are not intended to limitthe invention or claims in any conceivable manner but are intendedmerely to be informative of some of the objects, features, andadvantages of the present invention. In fact, these and yet otherobjects, features, and advantages of the present invention will becomeapparent from the drawings, the descriptions given herein, and theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram for a bubble measuring instrument in accordwith the present invention;

FIG. 2 is a frequency versus amplitude plot that discloses a beatfrequency signal in accord with the present invention;

FIG. 3 is a diagram showing relationships between pump frequency,repetition frequency, beat frequency, and image frequency;

FIG. 4 is a schematic view of a frequency domain beat signal that showsthe absence and presence of a beat signal at a preselected frequencyrelated to the absence and presence of a bubble;

FIG. 5 is a computer screen generated by software showing a virtualinstrument front panel for a bubble measuring instrument in accord withthe present invention;

FIG. 6 is a block diagram schematic of a video microscopy system forindependently counting and sizing bubbles to verify results from thebubble measuring instrument of the present invention;

FIG. 7 is an isometric view of a phantom tissue assembly for modelinganatomy including skin, tissue, blood flow, and bone to permitsimulation of an in-vivo measurement environment;

FIG. 8 is a schematic view of an oscilloscope screen showing ahigh-frequency return signal from a measurement volume wherein no bubbleis present;

FIG. 9 is a schematic view of an oscilloscope screen showing ahigh-frequency return signal from a measurement volume wherein a bubbleis present;

FIG. 10 is a comparison of video microscopy analysis of bubblepopulation to that of the bubble measuring instrument in accord with thepresent invention for a setting of 120 to 160 micrometers for bubblesize range to be measured;

FIG. 11 is a comparison of video microscopy analysis of bubblepopulation to that of the bubble measuring instrument in accord with thepresent invention for a setting of 97 micrometers for the bubble sizerange to be measured;

FIG. 12 is an isometric view of an in-vivo transducer fixture in accordwith the present invention; and

FIG. 13 provides an embodiment of one bubble producing system in accordwith the present invention.

While the present invention will be described in connection withpresently preferred embodiments, it will be understood that it is notintended to limit the invention to those embodiments. On the contrary,it is intended to cover all alternatives, modifications, and equivalentsincluded within the spirit of the invention and as defined in theappended claims.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, and more particularly to FIG. 1, thepresent invention discloses a non-invasive, in-vivo instrument 10 fordetecting, classifying and sizing gaseous emboli. Gaseous emboli whichresult in decompression sickness (DCS) pose a serious risk of injury toaviators, astronauts, divers and other individuals who are exposed tovarying environmental pressure. Gaseous emboli are also a seriouscomplication of cardiopulmonary bypass, surgical air introduced duringcardiotomy, and heart valve replacement resulting in cavitation bubbles.Instrument 10 will significantly aid research on DCS and other gaseousembolic events by providing a new capability to distinguish betweengaseous and thrombotic emboli, and to size gaseous emboli in anoninvasive way.

The fundamental basis of the instrument is the use of ultrasound toexcite the resonant behavior of the gaseous emboli. The resonantbehavior is a function of the gaseous bubble diameter and is wellcharacterized analytically so that little or no instrument calibrationis required. Nongaseous emboli do not display the same resonantcharacteristics as gaseous emboli, thus providing a means fordistinguishing between the two types of emboli. Instrument 10 consistsof low-frequency pump transducer 12 to excite the fundamental vibrationmode of the bubbles as indicated at 16, high-frequency imagingtransducer 14 to observe the resonance, and signal processing andanalysis equipment discussed in more detail hereinafter.

To interrogate a selected measurement volume 18 for a particular bubblesize, a pump frequency is selected, corresponding to a specific bubblesize, and the field is simultaneously insonified with the pump signaland the image signal. Because the bubbles act as nonlinear mixers, ahigh-frequency return signal is produced with sidebands at plus andminus the pump frequency, if and only if, there is a gaseous bubble witha fundamental resonant frequency corresponding to the pump transducerfrequency. The range of bubble sizes of interest may be scanned byvarying the pump frequency over the appropriate range. For variousreasons, we operate the instrument in a pulse/echo mode which addsseveral complications to the signal processing, but the benefits ofoperating in this mode are significant.

Instrument 10 exploits the resonant behavior of gaseous emboli. Thisbehavior is a strong function of bubble diameter and is well predictedtheoretically. As a consequence, by basing the instrument on theresonant behavior of the bubbles, it is possible to have an instrumentwhich provides excellent resolution and requires little or nocalibration. The theoretical relationships between bubble diameter,resonant frequency, and damping of the volume mode for a bubble of givendiameter have been known for some time and are given by:$f_{0} = {2\pi \sqrt{\frac{{3{\gamma \left( {P_{0} + \frac{2\sigma}{R_{0}}} \right)}} - \frac{2\sigma}{R_{0}}}{\rho \quad R_{0}^{2}}}}$

where,

f₀=first resonant frequency,

R₀=nominal bubble radius,

ρ=suspending mass density,

P₀=ambient pressure, and

σ=surface tension.

From this relationship, it can be determined that bubbles having adiameter in the range of 40 :m to 400 :m have resonant frequencies inthe range of 200 kHz to 20 kHz, respectively.

In accord with the present invention, low-frequency pump transducer 12is operated in a continuous mode at pump frequency f_(p)=T_(p)/2B, andthe sound pressure level on bubble 16 due to pump transducer 12 is knownto be:$P_{bp} \propto {\cos \quad {\omega_{p}\left( {t - \frac{r_{p}}{c}} \right)}}$

where,

r_(p)=distance between pump transducer 12 and bubble 16, and

c=speed of sound in the surrounding medium 18.

Bubble measuring instrument 10 employs low-frequency pump transducer 12made by a company of the name of Sonic Concepts. A single,high-frequency, 1″ spherically focused, 2.25 MHZ image/receivetransducer 14 was used in a preferred embodiment and is made by acompany of the name of Panametrics.

The pump frequency may preferably be stepped over the range of bubbleresonant frequencies of interest. In one preferred embodiment, the pumptransducer was driven by function generator 20, such as a HP33120-Afunction generator, and high-voltage amplifier 22, such as a Krohn-Hite7500 Amplifier. At each step, high-frequency 2.25 MHZ image transducer14 is preferably pulsed at a particular repetition rate via ultrasonicpulser 24, such as a Matec 310 pulser. In a preferred embodiment of theinvention, the image frequency is set a priori and is not changed ateach step of the pump frequency. The pulse repetition rate of pulser 24is on the order of 10 kHz and is under software control by the dataacquisition system, such as computer 26. The pulse repetition rate maybe adjusted through the virtual panel front of the software shown inFIG. 5 as discussed in more detail hereinafter. Specific restrictions onthe selection of the repetition rate and pump frequency in accord with apreferred embodiment of the present invention are discussed below. Thusas stated above, pulse repetition rate and pump frequency are preferablycontrolled by the computer.

High-frequency image/receive transducer 14 is preferably operated inpulse/echo mode and is driven in pulses or frequency bursts 34 (See FIG.3) of a sinusoidal signal of duration θ with each burst 34 being atimage frequency f_(i)=T_(i)/2B. The burst repetition frequencyf_(r)=1/T_(r). The sound pressure level on bubble 16 due to imagetransducer 14 is given by:$P_{bi} \propto {\sum\limits_{n = 1}^{\infty}\quad {{\underset{\tau}{\Pi}\left( {t - \frac{r_{i}}{c} - {nT}_{r}} \right)}\cos \quad {\omega_{i}\left( {t - \frac{r_{i}}{c}} \right)}}}$

where,

r_(i)=the distance between image transducer 14, and

c=speed of sound in the surrounding medium 18.

High-frequency image/receive transducer 14 receives a return signal thatis preferably amplified by amplifier 28, such as a Matec 605 ultrasonicreceive amplifier, and multiplied with multiplier 30 by the referencesignal produced by continuous wave oscillator 32 at the image frequency.The reference signal at the image frequency from oscillator 32 may begiven as:

X _(R1)=cos ω_(i) t.

Two possible cases occur for the received signals. In one case, thebubble acts linearly and the two insonifying signals of pump transducer12 and image transducer 14 do not interact at the bubble 16 surface. Inthe second case, bubble 16 acts as a nonlinear mixer. The linear casecorresponds to a nonresonant bubble, and the nonlinear case correspondsto a resonant bubble. For the linear case, the received sound pressurelevel is:$X_{{ri}\quad {linear}} \propto {\sum\limits_{n = 1}^{\infty}\quad {{\underset{\tau}{\Pi}\left( {t - \frac{2r_{i}}{c} - {nT}_{r}} \right)}\cos \quad {{\omega_{i}\left( {t - \frac{2r_{i}}{c}} \right)}.}}}$

The effect of the pumping frequency is neglected because the sensitivityof high-frequency transducer 14 at the much lower pump frequency isnearly zero. For the nonlinear case, bubble 16 acts like a mixer leadingto a received signal composed of harmonics of the pump frequency, imagefrequency, and sideband frequencies of f_(i)∀f_(p), f_(i)∀2f_(p), etc.If bubble 16 is assumed to be a quadratic mixer, as a firstapproximation, then the received high-frequency signal from bubble 16 asdetected by high-frequency transducer 14 is approximated by:$X_{{rih}\quad {nonlinear}} \propto {\sum\limits_{n = 1}^{\infty}\quad {{\underset{r}{\Pi}\left( {t - \frac{2r_{i}}{c} - {nT}_{r}} \right)}{\cos \left( {{\omega_{i}t} - \Phi} \right)}{\cos \left( {{\omega_{p}t} - \varphi} \right)}}}$

where,${\Phi = {\omega_{i}\left( \frac{2r_{i}}{c} \right)}},{{{and}\quad \varphi} = {\omega_{p}\left( \frac{r_{i} + r_{p}}{c} \right)}}$

The nonlinear behavior of the bubble creates spectral lines atf_(i)∀f_(p) as indicated in FIG. 2 which is discussed subsequently. Themagnitude of these sideband signals is maximum when the pump frequencycorresponds to the bubble resonant frequency.

In a preferred embodiment of the invention, two restrictions are placedon the repetition frequency f_(r) of the image signal controlled bycomputer 26 as effected by pulser 24. The first restriction is toeliminate range ambiguity. For this purpose, the repetition rate ischosen so that: $f_{r} < \frac{c}{\left( {2r_{\max}} \right)}$

where r_(max) is the maximum distance between transducers 12 and 14 andthe target such as bubble 16. The second restriction is that therepetition rate or frequency f_(r) cannot be an integer multiple of thepump frequency f_(p). That is: $f_{r} \neq \frac{f_{p}}{N}$

where N is any integer. The basis for this restriction is that asuperposition of the nonlinear and linear responses seen above resultswhen f_(r)=f_(p)/N. In this case, the magnitude of the sidebands iszero. This restriction on the repetition rate or frequency f_(r) ispreferably handled by software of computer 26 which preferably controlsthe repetition rate or frequency f_(r) and pump frequency f_(p) based onthe desired bubble size range to be scanned. As the repetition rate forthe present invention is preferably chosen significantly lower than theresonant frequency of any bubbles of interest, undersampling occursresulting in aliasing and a beat signal at beat frequency f_(b). FIG. 3shows a representative view of the relationship between some of thesignals discussed above with respect to time. Bursts 34 are the imagingsignal output from image transducer 14 and are pulses produced at aburst repetition frequency f_(r) with each burst or pulse being ahigh-frequency sinusoidal signal at the image frequency f_(i). The timebetween each burst 34 as indicated at 40 is I_(r). Signal 36 is the pumpsignal at frequency f_(p). The beat signal 38 is produced in thenon-linear case of a resonant bubble at frequency f_(b). As discussed inmore detail hereinafter, the frequency f_(b) of the beat signal can becontrolled by suitable adjustment of f_(p) and f_(r).

For nonlinear targets (resonant bubbles), the output of the multiplier30, which multiplies the received signal and the signal of oscillator 32for demodulation purposes, is given by:$Z_{{ib}\quad {nonlinear}} \propto {\sum\limits_{n = 1}^{\infty}\quad {{\underset{r}{\Pi}\left( {t - \frac{2r_{i}}{c} - {nT}_{r}} \right)}{\cos \left( {{\omega_{i}t} - \Phi} \right)}{\cos \left( {{\omega_{p}t} - \varphi} \right)}\cos \quad \omega_{i}t}}$

Because the duration A_(r) is large compared to ½f_(i), it is possibleto filter out the high-frequency components present at the output of themultiplier 30 by lowpass filter 42 to give a lowpass filter output of:$X_{{rih}\quad {nonlinear}} \propto {\sum{{\underset{r}{\Pi}\left( {t - \frac{2r_{i}}{c} - {nT}_{r}} \right)}\cos \quad {{{\Phi cos}\left( {{\omega_{p}t} - \varphi} \right)}.}}}$

For linear targets (nonresonant bubbles) the output of the multiplier 30is:$Z_{1\quad {linear}} \propto {\sum\limits_{n = 1}^{\infty}\quad {{\underset{\tau}{\Pi}\left( {t - \frac{2r_{i}}{c} - {nT}_{r}} \right)}{\cos \left( {{\omega_{i}t} - \Phi} \right)}\cos \quad \omega_{i}{t.}}}$

and the output of lowpass filter 42 is given by:${Z_{1\quad {linear}} \propto {\sum\limits_{n = 1}^{\infty}\quad {{\underset{\tau}{\Pi}\left( {t - \frac{2r_{i}}{c} - {nT}_{r}} \right)}\cos \quad {\Phi.}}}}\quad$

In one preferred embodiment of instrument 10, lowpass filter 42 has acutoff frequency of 250 kHz. The linear and nonlinear responses of thesystem after demodulation are depicted in FIG. 2 in the frequency andtime domains. For the linear response, we see that the output signal isa low-frequency signal sampled at the repetition frequency f_(r). Asdiscussed above, because the repetition rate f_(r) is significantlylower than the resonant frequency of any bubbles of interest, we areundersampling, resulting in aliasing and a beat signal of frequencyf_(b). The frequency f_(b) of the beat signal is given by:

f _(b) =|Nf _(b) +f _(p)|_(min).

If f_(p)=Nf_(r), then the beat frequency f_(b) is zero, and there is noway to distinguish between linear and nonlinear targets. Consequently,as discussed previously, f_(r) is chosen such that f_(r)f_(p)/N. Sinceboth f_(p) and f_(r) are independently controlled, this is not aproblem. In fact, in accord with the preferred embodiment of the presentinvention, computer 26 selects the proper pulse repetition rate f_(r)for each pump frequency f_(p) to ensure that the beat frequency isalways located at a predetermined frequency. This feature allowssimplified electronic implementation and is considered highlyadvantageous.

The beat signal can be extracted from the demodulated, lowpass filteredreturn signal, i.e., the output of lowpass filter 42, by appropriatelyrange gating and using sample and hold unit 44. Sample and hold unit 44convolves the sampled signal and the gate function of duration T_(r)/2.The output of sample and hold unit 44 is then lowpass filtered bylowpass filter 46 to eliminate high-frequency spectral components whichare artifacts of the signal processing. The signal is also highpassfiltered to eliminate low-frequency Doppler effects by low pass filter48. However, it will also be noted that the Doppler signal is providedon line 50 is the output of lowpass filter 46 and may be viewed onoscilloscope 53. The Doppler signal is used by the software of computer26 as may be indicated on virtual panel front 58 discussed hereinafter.The Doppler signal on line 50 may be used to indicate when a bubble, orother emboli, has passed through the measurement volume.

In accord with a presently preferred embodiment, the pulse or burstrepetition rate f_(r) and the pump frequency f_(p) are controlled bycomputer 26 such that the beat signal frequency f_(b) is at a constantfrequency. In a preferred embodiment of instrument 10, 4 kHz wasselected. Consequently lowpass filter 46 and highpass filter 48 can bechosen closely around 4 kHz in this embodiment of the invention. Forthis preferred embodiment, 5 kHz was chosen for lowpass filter 46 and 3kHz was chosen for highpass filter 48. This design results in excellentnoise rejection and selectivity.

FIG. 4 is a frequency domain signal, preferably a fast Fourier transformor FFT signal, that is processed by software from the output data ofhighpass filter 48. The FFT signal may be viewed in the virtual panelfront 58 discussed hereinafter. The raw signal output of highpass filter48 from which the FFT signal is produced is found on line 52 and may beviewed by oscilloscope 53. FIG. 4 shows the case where a resonant bubbleis present with FFT curve 54, which has a center peak at 4 kHz, and forthe case where a resonant bubble is not present with FFT curve 56. Theamplitude and frequency band for the FFT signal are selectable fordetermining whether the instrument counts a bubble or not from virtualpanel front 58.

FIG. 5 discloses virtual instrument front 58 as might be seen oncomputer monitor 60 from FIG. 1 in accord with software used foroperation of the present invention. With the exception of the setting ofsample and hold gate position as operated with elements 44 and delaygenerator 62 from FIG. 1, and the pump signal amplitude adjustment asper element 22 from FIG. 1, instrument 10 in a preferred embodiment iswholly controlled from virtual front panel 58 so as to be under softwarecontrol. The software controls of panel 58 include setting the pumpsweep frequency range (and hence the bubble sizing range) and stepfrequency increment, the repetition frequency, and the thresholds onbubble detection. Trigger in line 68 from computer 26 to ultrasonicpulser 24 sets the pulse repetition rate f_(r). In one preferredembodiment of the invention, bubble sizing instrument 10 is controlledby and data are collected and processed on a Power Macintosh computerwith National Instruments data acquisition and GPIB cards.

A signal and hold gate adjust signal is available at line 66 for view onoscilloscope 53 that is used to adjust the gate location at which theamplified, mixed, 250 KHz lowpass filtered output signal from lowpassfilter 42 is sampled by sample and hold unit 44. This signal is used inconjunction with output from lowpass filter 42 at line 70 and displayedon oscilloscope 53 with gate signal imposed therewith to set the sampleand hold gate location. The sample and hold gate is set such that thepeak of the bubble response envelope is held. Because the sampling takesplace instantaneously, i.e. at a set time delay from the pulse, itcannot capture the responses of all bubbles that flow through a vessel.Thus, the gate is preferably set at the position of the greatest amountof bubble activity, which is usually the center of the vessel.

The pump amplitude may be preferably set using high voltage amplifier22, which may be a Krohn-Hite 7500 unit. The amplitude should be sethigh enough to resonate the bubbles. However, the pump transducer signalcan couple with that of the image transducer 14 if its amplitude is settoo high.

Referring to FIG. 5, in a preferred embodiment, the user determines therange of bubble sizes to interrogate by setting the start, stop, andstep frequencies on middle left box 64 of the control panel such as inthe present example where start frequency is at box 72, end frequency atbox 74 and increment or step frequency at box 76. As the pump transducersweeps through this frequency range, only one bubble size isinterrogated at a time. The Number of FFT's per Frequency Increment box78 determines the time spent at each frequency. Thus, there is atradeoff between sweeping through the bubble size range quickly byjumping over bubble sizes and sweeping through slowly, interrogatingmany more bubble sizes within the range.

The repetition rate is input at the lower left corner of the controlpanel at box 80 and sets the rate at which the bubbles are pulsed withthe imaging signal. This is a nominal frequency such as 10 kHz or arange from about 4 kHz to 12 kHz as the actual repetition rate iscalculated and varies around this frequency. In bubble sizingexperiments, we have found that noise in the bubble sizing signal couldbe minimized by adjusting the repetition rate to some optimum value,which was suited for that particular acoustic environment. As discussedabove, the repetition rate f_(r) is used to control beat frequency f_(b)which is optimally set at 4 kHz for the present setup as indicated at82.

Five plots are displayed on screens or windows on the control panel. Thetwo on the left side are real-time plots of the Doppler signal at 84 andthe Bubble Data signal at 86. The Doppler signal gives a visualindication of the presence of bubbles. It is nominally flat when nobubbles pass by the measurement volume and fluctuates in amplitude whenbubbles are present. The bubble data signal is the final output of thesignal processing electronics at line 52. From this signal, the softwarecalculates a frequency domain signal or FFT signal such as shown in FIG.4 and displays it in center plot 88. Bubbles of a given size are deemeddetected when the amplitude of the FFT signal exceeds the amplitude FFTThreshold setting and occurs at a frequency within the ThresholdFrequency Span. These two thresholds settings are located in the middleleft box on the control panel at 90. The two plots on the right-handside of the control panel display the bubble sizing results. The topright plot at 92 displays the real-time histogram of whether a bubble ofa given size has been detected, and the plot beneath it at 94 displaysthe bubble sizing histogram from the last sweep.

While running a bubble sizing test, the data may be concurrently writtento a file using the inputs on the lower right side of the control panel.Prior to running a test, the user inputs a file name and enables theSave File switch.

Various means were used for bubble production. Hydrolysis techniques(not shown) easily provides large numbers of small (˜20 :m diameter)bubbles. However, it is difficult to produce bubbles much larger than100 :m diameter via this technique. The hydrolysis technique also tendsto produce a dense cloud with a broad distribution of bubble sizes.Consequently, an alternative approach was required to produce bubblesover the entire 40 :m to 400 :m diameter size range of interest herewith both narrow and wide size distributions as required.

Using a bubble producing system such as system 110 shown in FIG. 13,bubbles were also produced by blowing helium or air through a smalldrawn capillary tube. To control the size of the bubbles, a water jetwas directed upward at the tip of the capillary tube. By controlling thesupply pressure to the capillary tube and the jet velocity, we were ableto control the bubble size and the number of bubbles produced. We wereoften able to produce bubbles on the order of 20 :m in diameter withthis technique, and reliably we could produce them down to the 30 :m to40 :m in diameter. In the 60 :m diameter range and above, we were ableto produce relatively mono-disperse bubble clouds with a narrow sizedistributions. When smaller bubble sizes were produced, thedistributions tended to be broader.

Bubbles were sized using video microscopy and image analysis software.The setup is shown schematically in FIG. 6. The system consists of a CCDvideo camera, 50×fixed lens, monitor, strobe light, light filter, andcomputer running NIH Image software. The resolution of the system basedon the resulting pixel size is approximately 4 :m. Video data iscollected on a Power Macintosh computer with its built in frame grabbingcapability. Using software, such as NIH Image Software, the bubble sizesare measured on a number of statistically independent frames (e.g., thesame bubble was never measured twice in two different frames) todetermine the size distribution for a given set up of the bubble blower.

Initial work was conducted in a large tank with free-floating bubbles.However, to more accurately model the in-vivo testing situation inaccord with the objectives of the present invention, several tissuephantoms were constructed for use in connection with the videomicroscopy system of FIG. 6 and the bubble generating apparatus of FIG.13. Tissue phantoms, an example of which is shown in FIG. 7, wereconstructed modeling both the anatomy of the upper human leg and theinner, upper portion of a canine rear leg. The human leg tissue phantomconsisted of a Plexiglas support structure, a tissue mimicking gel, andskin mimicking polymer covering. A Plexiglas rod was embedded in thephantom to model the femur. A tubing material is preferably used tomodel the femoral vasculature having ultrasonic properties similar tothose of actual blood vessels as known to those of skill in the art.Coupling to the vessel was provided for by filling the phantom withwater, and the skin was modeled with a thin latex sheet. The bone behindthe vessel was modeled with PVC plastic. All of the in-vitro testresults presented herein were obtained with the canine tissue phantomsince this was most representative of our initial in-vivo application.

Bubbles were produced in bubble generating apparatus 110 in an openwater tank as indicated in FIG. 13 for most of the testing over therange of bubbles from 40 :m to 400 :m. For bubbles on the order of 20 :mand smaller, a hydrolysis system (not shown) was found to be morereliable. However, apparatus 110 was satisfactory for producing bubblesover the range of sizes of initial interest.

It will be noted that generating bubbles of different sizes was adifficult task. Glass tubes of appropriate sizes were drawing and usedand it was found that jagged end tubes were more reliable than polishedor smooth ending glass tubes. For certain ranges of bubbles, significanttrial and error was required to produce the correct size glass tube.Compressed air was directed through the glass tubes and also used toproduce a water jet. By controlling the supply pressure to the capillaryglass tube and the jet pressure, the bubble size and number of bubblesproduced could be controlled. The bubbles were entrained in an upwardflow through sight tube 112 such as the sight tube of FIG. 6, where theycould be sized optically. The bubbles were then directed through thesimulated vasculature 116, such as that shown in FIG. 7. To provide alife-like in-vitro simulation, the flow was driven by peristaltic pump118 at velocities in the range of two cm/sec to ten cm/sec to mimicheart flow.

An objective of the present invention was to show that the ultrasonicbubble sizing instrument produced results which could be confirmed byindependent video microscopy. As such, the main result of the in-vitrotesting is a series of histogram plots comparing the output of thebubble sizing instrument to the size distributions measured by videomicroscopy. Representative examples of results for sample bubble sizedistributions are shown in FIG. 10 and FIG. 11. Intermediate resultssuch as RF return signals from the measurement volume are shown in FIG.8 and FIG. 9 and the spectral decompositions of the processed returnsignals showing the beat frequency from bubbles, and the lack of a beatfrequency for non-resonant bubbles in FIG. 4 are also presented.

In more detail, FIG. 8 and FIG. 9 illustrates the high-frequency returnsignals from the measurement volume without and with a bubble present inthe volume as might be seen on oscilloscope 53 selecting the signal online 96 using a tissue phantom such as that of FIG. 7. Bubble echo 98 isseen in FIG. 9. The results are essentially independent of the bubblesize although larger bubbles tend to create echoes with largermagnitude. FIG. 8 and FIG. 9 clearly illustrate the reflections from thenear and far walls of the simulated blood vessel. The reflections alsoprovide a convenient means for aligning high-frequency transducer 14 invitro and in vivo.

FIG. 4 presents the spectrum of processed return signals 54 and 56 fromthe high-frequency image transducer as discussed previously. The plot iscentered at 4 kHz since the beat frequency f_(b) is preferably selectedas 4 kHz and the presence of the beat signal at this frequency indicatesthere is a resonant bubble. As discussed above, the pulse repetitionrate f_(r) and pump frequency f_(p) are preferably selected to producethis beat frequency. This feature of the invention allows us tothreshold over a single small frequency range, reducing the noise andimproving the selectivity of the instrument. FIG. 4 clearly shows areturn signal 54 with a strong 4 kHz component, corresponding to aresonant bubble, and another return signal 56 without a strong 4 kHzcomponent, corresponding to the absence of a resonant bubble. Asdiscussed above, such signals may be displayed at 88 of virtualinstrument panel 58 with the amplitude and frequency cut off rangesselected at 90 so instrument 10 will count the signal as a bubble ornot.

FIG. 10 and FIG. 11 present several comparisons of the bubble sizedistributions as determined by the bubble sizing instrument and theindependent video microscopy sizing technique. In general tests, it wasfound that the comparison between the size distributions between the twotechniques is superb.

Extending the range of the instrument above 200 :m requires the use ofanother pump transducer which can operate at frequencies below 30 kHz.Instrument 10 was successfully used with such a transducer, but becausethese large bubbles are not likely to be of much interest in DCSresearch not much work was done in this range. Extending the range ofthe instrument below 30 :m bubble diameters is also possible by usinganother pump transducer which can operate at frequencies above 200 kHz.Based on the signal-to-noise ratio we observed at 30 :m bubblediameters, we have every reason to believe that with a suitable pumptransducer, it would be possible to size bubbles much smaller than 30 :mwith instrument 10. Instrument 10 works very well even if the bubblesize distribution is broad.

For in-vivo testing a fixture, such as fixture 102, is preferably usedto hold the imaging and pump transducers as indicated in FIG. 12.Fixture 102 allows for the independent height adjustment of the imagetransducer so that it can be optimally positioned at its focal distance.The fixture also allows for an independent angle adjustment on the pumptransducer. However, the pump transducer is relatively omni-directional,and its alignment is not critical. Transducer holder 104 can be packedwith ultrasonic coupling gel, and there is an adjustable base portionwhich allows one to alter the height of both transducers and contain thecoupling gel. Transducer holder 104 is held by a multi-degree-of-freedomsupport arm 106. The preliminary results provided a good demonstrationof the device in vivo, which was the primary objective of this activity.Because fixture 102 is designed for external use, other design criteriawould be required for use internally. The optimum focusing distance forthe image transducer of a presently preferred embodiment is 0.125inches. Pump transducer 12 is preferably directed slightly upstream ofthe focusing point for image transducer 14 so as to be resonatingbubbles before the point where they are insonified by image transducer14.

It will be understood that numerous variations of the invention may bemade. Several variations are listed below but are not considered to beexhaustive of the possible variations but simply show examples.

As mentioned above, transducer fixture 102 was intended for the externalfemoral vein location and works well there. However, a smaller, morecompact fixture could easily be fabricated if desired for internal useas may be desired for testing purposes. A different, more flexiblesupport arm would be required for internal purposes as well.

General testing could determine the best type of coupling such aswhether or not a coupling gel bag should be used instead of freestanding gel. The get bags might prove to be a better approach in thatthere will likely be less entrapped air bubbles, and might also be a lotneater.

A Doppler audio signal could be used to aid alignment. This would be arelatively straightforward enhancement, because a pseudo-Doppler signalis a byproduct of the signal processing. This would likely be veryhelpful, especially if it would allow the user to hear the blood flowingin a vessel.

It would be desirable to extend bubble size range below 30 :m diameterbubbles. The original target lower bound for the range of the bubblesizing instrument was 40 :m diameter bubbles. We exceeded this targetand demonstrated operation down to 30 :m diameter bubbles. Preliminaryin-vivo test results indicate that there are DCS bubbles with 30 :mdiameter. Results at 30 :m diameter give us every indication that anappropriate higher frequency pump transducer is all that is needed alongwith-in-vitro testing to verify the operation of the instrument atsmaller bubble diameters.

It would also be desirable to build an artery/vein cuff. DCS researchwould be aided by an artery or vein cuff that would aid alignment of thesystem directly on a desired excised vessel such as the vena cava orpulmonary artery.

It would also be desirable to include a transesophogeal probe. It wouldbe possible to take a standard transesophogeal ultrasound probe andintegrate the bubble sizing transducers into it and the ultrasoundmachine so that one could easily target a specific vessel or region oftissue using the ultrasound machine. A reticule would be used to selectthe target volume on the ultrasound image as is done now for the Doppleron a standard ultrasound system.

The pulmonary artery is of significant interest in DCS research andin-suit monitoring of astronauts for DCS precursor bubbles. Therefore,instrument 10 could be applied to monitoring of DCS bubbles in thepulmonary artery.

Instrument 10 could also be modified for use transcutaneously. Theconcepts of instrument 10 may also be suitable for a transcatheteroperation with appropriate transducers and adjustments to frequenciesbased on the limitations of such transducers.

Artificial heart valves are known to sometimes generate emboli. It isnot known if these emboli are thrombic or gaseous emboli produced bycavitation. Consequently, the bubble monitoring instrument could be usedto classify these emboli.

The formation of the extravascular bubbles is of significant interest toDCS researchers. The bubble sizing instrument disclosed herein couldvery likely be applied to the study of extravascular bubbles. Althoughit has not been specifically tested in this scenario, it is likely thatextravascular bubbles will display resonant behavior very similar tothat of intravascular bubbles. Since the primary interest inextravascular bubble research is to determine whether or not the bubblesare forming, it is not important that the resonant behavior be exactlypredicted or known, just that the bubbles do resonate so that they canbe detected. Existing ultrasound imaging techniques do not reveal theirpresence.

In a brief summary, the general procedure for running the instrument isto power up the electronics and start the software, align thetransducers, set the sample and hold gate position, set the pumpamplitude, adjust the software controls, and finally take bubble sizingdata. With the exception of the setting of the sample and hold gateposition and the pump amplitude discussed herein before, a preferredembodiment of instrument 10 is wholly controlled via inputs on thevirtual front panel 58 shown in FIG. 5. A range of bubble sizes to beinvestigated is selected. The time spent surveying the range isselected. The relative frequency band and amplitude of the FFT signal isselected to indicate the presence of a bubble. Numerous other parametersmay also be selected depending on the particular investigation. Theinformation is then acquired and can be displayed to indicate quantitiesof bubbles and their relative sizes which were detected in themeasurement volume.

Thus, while the preferred embodiment of the bubble monitoring apparatusand methods are disclosed in accord with the law requiring disclosure ofthe presently preferred embodiment of the invention, other embodimentsof the disclosed concepts may also be used. Therefore, the foregoingdisclosure and description of the invention are illustrative andexplanatory thereof, and various changes in the method steps and alsothe details of the apparatus may be made within the scope of theappended claims without departing from the spirit of the invention.

What is claimed is:
 1. An apparatus for monitoring bubbles in a selectedvolume, comprising: a pump transducer; a controller for said pumptransducer operable for producing a signal at a first frequency fromsaid pump transducer, said first frequency being selectable over a rangeof frequencies; an image transducer; a reference signal generator forproducing a reference signal at a second frequency higher than saidfirst frequency; a pulser for producing a pulsed signal output from saidimage transducer at said second frequency; a receiver for detecting areturn signal from said image transducer; and a multiplier formultiplying said return signal with said reference signal to produce amultiplier output signal.
 2. The apparatus of claim 1, furthercomprising: a first lowpass filter for filtering said multiplier outputsignal to produce a first filtered signal.
 3. The apparatus of claim 1,further comprising: a sample and hold circuit for receiving said firstfiltered signal to produce a beat signal.
 4. The apparatus of claim 3,further comprising: a second lowpass filter for producing said beatsignal.
 5. The apparatus of claim 3, further comprising: a highpassfilter for producing said beat signal.
 6. The apparatus of claim 3,further comprising: a level detector for detecting a bubble from saidbeat signal.
 7. The apparatus of claim 1, further comprising: a pulsercontroller for said pulser for controlling a repetition frequency ofsaid pulsed signal output.
 8. The apparatus of claim 7, wherein saidpulser controller is software operable for selecting of said repetitionfrequency.
 9. The apparatus of claim 7, wherein said pulser controllervaries said repetition frequency based on said first frequency.
 10. Theapparatus of claim 7, wherein said pulser controller is programmable forvarying said repetition frequency to maintain a constant frequency of abeat signal contained in said return signal.
 11. An apparatus formonitoring bubbles in a selected volume, comprising: a pump transducer;a controller for said pump transducer operable for producing a signal ata first frequency from said pump transducer, said first frequency beingselectable over a range of frequencies; an image transducer; a referencesignal generator for producing a reference signal at a second frequencyhigher than said first frequency; a pulser for producing a pulsed signaloutput from said image transducer at said second frequency; a pulsercontroller for controlling a repetition frequency of said pulsed signaloutput produced by said pulser, said pulser controller operating to varysaid repetition frequency as a function of said first frequency; and areceiver for detecting a return signal from said image transducer. 12.The apparatus of claim 11, further comprising: a multiplier formultiplying said return signal with said reference signal to produce amultiplier output signal.
 13. The apparatus of claim 11, furthercomprising: a sample and hold circuit for detecting a beat signal. 14.The apparatus of claim 11, further comprising: at least one filter fordetecting a beat signal with a beat signal frequency.
 15. The apparatusof claim 14, said pulser controller being operable for varying saidrepetition frequency to maintain said beat signal frequency at aselected frequency.